In an aluminum alloy composite material for brazing in which a core material is cladded with a brazing material on one surface and a cladding material on the other surface, the strength of the core material is improved by adding Mg to the core material. However, when the Mg content in the core material exceeds 0.2% by mass (0.2 mass %), the brazeability is extremely lowered. Therefore, addition of Mg to the core material is not preferable.
An aluminum alloy composite material is proposed which can be made thin without hindering the corrosion resistance, the brazeability, and the like (see Japanese Patent Laid-Open Publication Nos. Hei 8-283891 and Hei 11-61306). Especially, in the aluminum alloy composite material described in Japanese Patent Laid-Open Publication No. Hei 8-283891, the cladding material contains Mg of 0.3 to 3 mass % and Zn of 2.2 to 5 mass %. This aluminum alloy composite material improves the strength of the cladding material by the addition of Mg.
Moreover, an aluminum alloy composite material is proposed in which the strength of the cladding material is improved by adding Mn and Si to the cladding material, instead of Mg (see Japanese Patent Laid-Open Publication No. Hei 11-61306).
However, the conventional techniques described above have the following disadvantages. First, the aluminum alloy composite material described in Japanese Patent Laid-Open Publication No. Hei 8-283891 aims to improve the strength by adding Mg to the cladding material. However, when the aluminum alloy composite material is made thinner, Mg is diffused by heat during brazing and reaches a surface of the brazing material through the core material, thus deteriorating the brazeability. Therefore, it is necessary to form the core material to be thick in order to ensure a desired brazeability. That is, making the aluminum composite material thin is limited. Moreover, in this conventional aluminum alloy composite material, the brazeability on the cladding material side is inferior to that of the brazing material. Therefore, this conventional aluminum alloy composite material cannot be used as a tube material that is brazed on the cladding material side.
On the other hand, the aluminum alloy described in Japanese Patent Laid-Open Publication No. Hei 11-61306 aims to improve the strength by adding Mn and Si to the cladding material. However, making the aluminum alloy composite material thinner without deteriorating the corrosion resistance and the like only by adding Mn and Si to the cladding material has a limitation. More specifically, in case of adding Si to the cladding material, precipitation of Al—Mn—Si precipitates at grain boundaries occurs during brazing. Thus, the corrosion resistance of the cladding material is deteriorated and a time period during which the cladding material serves as a sacrifice anode layer is shortened. This results in deterioration of the corrosion resistance of the cladding material on the inner-surface side (coolant side). Moreover, the core material usually contains Cu that is diffused into the cladding material during the brazing. Thus, precipitation of Al—Cu—Si precipitates or Al—Mn—Cu—Si precipitates occurs at the grain boundaries in the cladding material so as to deteriorate the corrosion resistance of the cladding material. This also causes deterioration of the corrosion resistance of the cladding material on the inner-surface side.